Six Low-Lying Isomers of C 11 H 8 Are Unidentified in the Laboratory–A Theoretical Study

Isomers of C 11 H 8 have been theoretically examined using density functional theory and coupled-cluster methods. The current investigation reveals that 2 a H-cyclopenta[ cd ]indene ( 2 ), 7-ethynyl-1H-indene ( 6 ), 4-ethynyl-1H-indene ( 7 ), 6-ethynyl-1 H -indene ( 8 ), 5-ethynyl-1 H -indene ( 9 ), and 7b H -cyclopenta[ cd ]indene ( 10 ) remain elusive till date in the laboratory. The puckered low-lying isomer 2 lies at 9 kJ mol –1 below the experimentally known molecule, cyclobuta[ de ]naphthalene ( 3 ), at the fc-CCSD(T)/cc-pVTZ//fc-CCSD(T)/cc-pVDZ level of theory. 2 lies at 36 kJ mol –1 above the thermodynamically most stable and experimentally known isomer, 1H-cyclopenta[ cd ]indene ( 1 ), at the same level. It is identified that 1,2-H transfer from 1 yields 2 H -cyclopenta[ cd ]indene ( 14 ) and subsequent 1,2-H shift from 14 yields 2 . Appropriate transition states have been identified, and intrinsic reaction coordinate calculations have been carried out at the B3LYP/6-311+G(d,p) level of theory. Recently, 1-ethynyl-1 H -indene ( 11 ) has been detected using synchrotron-based vacuum ultraviolet ionization mass spectrometry. 2-Ethynyl-1 H -indene ( 4 ) and 3-ethynyl-1 H -indene ( 5 ) have been synthetically characterized in the past. While the derivatives of 7b H -cyclopenta[ cd ]indene ( 10 ) have been isolated elsewhere, the parent compound remains unidentified till date in the laboratory. Although C 11 H 8 is a key elemental composition of astronomical interest for the formation of polycyclic aromatic hydrocarbons in the interstellar medium, none of its low-lying isomers have been characterized by rotational spectroscopy though they are having a permanent dipole moment (μ ≠ 0). Therefore, energetic and spectroscopic properties have been computed, and the present investigation necessitates new synthetic studies on C 11 H 8, in particular 2, 6–10, and also rotational spectroscopic studies on all low-lying isomers.